Need Some Dimensional Info

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  • Need Some Dimensional Info

    Here's the problem let's say you have a dim of 1.00 tolerance of +/- .001 and you use a gauge that meets the factor of 10 on the tolerance ( accuracy of .0002) can I accept the part if it reads 1.0011 or .9989. or even 1.0014 based on the tolerance being only 3 decimal places? Is there a spec or std that proves this.

    Thanks
    "A good design is the one that allows engineers the ability to change gracefully what they forgot to do right the first time!!!"

  • #2
    Some diehards will say yes and the reason is that the "real value" might actually be 1.0013 due to the error. Some will contend that you can round the number to the nearest decimal place(3). But the problem with the latter is that the dimension should probably have said 1.0000 +/-.001 because of the tolerance. You shouldn't have a tolerance of plus or minus 1 unit.

    As an inspector - reject it.
    Links to my utilities for PCDMIS

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    • #3
      That is exactly the issue, I only have 3 decimal places but we are rejecting the part based on 4 decimal places. Can I round the result down if it is out .0004 or less? I have been told this is in a spec somewhere but I have never seen it.
      "A good design is the one that allows engineers the ability to change gracefully what they forgot to do right the first time!!!"

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      • #4
        I dont see how you can round it. The 3 decimal places give you only +/- one unit. You would have to go to four in order to get the 10 to one rule that you indicated that you wanted. By rounding it, you are ignoring the 10 to one rule.

        .0001" represents 5% of your tolerance.
        Last edited by cmmguy; 10-31-2006, 01:22 PM.
        Links to my utilities for PCDMIS

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        • #5
          ASTM E 29 is Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
          Rounding procedures are covered in ASTM Manual 7.
          If you have the CMI Primer from ASQ, it covers rounding in the Inspection and Testing section.( Actually it quotes ASTM Manual 7)
          When in doubt, post code. A second set of eyes might see something you missed.
          sigpic

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          • #6
            I was always taught that the limit is the limit. Either you are in tol or not. So even if your tol is +/- .5 if you are .0001 over the high you are out of tol. If you want to take into account measurement uncertainty then you only use it to narrow the tol of what you will accept, but never to enlarge it. So if your tol is +/-.010 and your uncertainty is .0004, then you could narrow your tol to +/- .0096 but you could not grow it to +/-.0104. Of course I have also heard the following phrases often: "They won't check it, ship it.", "Let them prove it is out.", "What is a few tenths amoung friends?", etc.
            sigpic"Hated by Many, Loved by Few" _ A.B. - Stone brewery

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            • #7
              Originally posted by John Kingston View Post
              ASTM E 29 is Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
              Rounding procedures are covered in ASTM Manual 7.
              If you have the CMI Primer from ASQ, it covers rounding in the Inspection and Testing section.( Actually it quotes ASTM Manual 7)
              And the answer is....?

              Originally posted by Wes Cisco View Post
              ... If you want to take into account measurement uncertainty then you only use it to narrow the tol of what you will accept, but never to enlarge it. So if your tol is +/-.010 and your uncertainty is .0004, then you could narrow your tol to +/- .0096 but you could not grow it to +/-.0104
              Thank you, that is what I was trying to say earlier but you put it a much better way. Gage uncertainty always goes against you, never for you.
              Links to my utilities for PCDMIS

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              • #8
                Originally posted by Wes Cisco View Post
                If you want to take into account measurement uncertainty then you only use it to narrow the tol of what you will accept, but never to enlarge it. So if your tol is +/-.010 and your uncertainty is .0004, then you could narrow your tol to +/- .0096 but you could not grow it to +/-.0104.

                I agree with you Wes. We have the same discussion here all the time. The problem is how to determine the uncertainty. It has little to do with accuracy and resolution.

                When we have close tolerance measurements, we try to subtract the uncertainty from the tolerance band. Like you argue, we would us the +/-0.0096. That would guarantee us that the measurement is always within the print limits. But how do you know that the uncertainty is 0.0004"????

                Determining the uncertainty of the measurement can be hard. You have to look at your complete measuring plan (like how many PH10 angle rotations do you have), not just the machine.



                Jan.
                ***************************
                PC-DMIS/NC 2010MR3; 15 December 2010; running on 18 machine tools.
                Romer Infinite; PC-DMIS 2010 MR3; 15 December 2010.

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                • #9
                  Originally posted by Jan d. View Post
                  I agree with you Wes. We have the same discussion here all the time. The problem is how to determine the uncertainty. It has little to do with accuracy and resolution.

                  When we have close tolerance measurements, we try to subtract the uncertainty from the tolerance band. Like you argue, we would us the +/-0.0096. That would guarantee us that the measurement is always within the print limits. But how do you know that the uncertainty is 0.0004"????

                  Determining the uncertainty of the measurement can be hard. You have to look at your complete measuring plan (like how many PH10 angle rotations do you have), not just the machine.



                  Jan.
                  The calibration house should provide you with the uncertainty values for the gage. The GR&R evaluates the other parameters.
                  Links to my utilities for PCDMIS

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                  • #10
                    In Y14.5 paragraph 2.4 (page 25) says " All limits are absolute. Dimensional limits, regardless of the number of decimal places, are used as if they were continued with zeros......"

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                    • #11
                      [QUOTE=cmmguy;37700]And the answer is....?[QUOTE]

                      I didn't give a complete answer because every company uses its' own standard.
                      Are you allowed to round? Depends on the customer.
                      If you round away the .0002" will the part still function?
                      So many questions to ask. I just wanted to lead him to the specs. and let him (and his organization make a decision.)
                      When in doubt, post code. A second set of eyes might see something you missed.
                      sigpic

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                      • #12
                        Originally posted by Wes Cisco View Post
                        Of course I have also heard the following phrases often: "They won't check it, ship it.", "Let them prove it is out.", "What is a few tenths amoung friends?", etc.



                        When in doubt, ship it out!
                        sigpic Life is not a journey to the grave with the intention of arriving safely, but rather a skid in broadside, totally worn, proclaiming WOW What a ride!

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                        • #13
                          If in doubt, chuck it out )
                          Recently jumped from 3.5 Mr 2 CAD
                          to 2012 CAD++

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                          • #14
                            Originally posted by kingsld1 View Post
                            In Y14.5 paragraph 2.4 (page 25) says " All limits are absolute. Dimensional limits, regardless of the number of decimal places, are used as if they were continued with zeros......"
                            Thanks for all the info this is exactly what I needed for backup of my original position.
                            "A good design is the one that allows engineers the ability to change gracefully what they forgot to do right the first time!!!"

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